1. Field of the Invention
The present invention relates to a printer which prints out characters, bar codes and the like on a paper sheet positioned at a printing position determined in accordance with a mark provided on the paper sheet or a base sheet on which the paper sheet is provided.
2. Description of the Related Art
A conventional printer, e.g., a label printer for printing characters or bar codes onto a plurality of label sheets which are adhered in predetermined intervals on the base sheet uses a transmission type sensor of optical transmission type and detects thick portions (i.e., label portions) and thin portions (i.e., gap portions) of the base sheet which are exposed between the labels, on the bases of detection levels sensed by the sensor.
In the conventional label printer, a gap between two adjacent label sheets is used as a mark for determining a printing start position of a label sheet. An output level of the transmission type sensor is detected each time one step driving motion of a stepping motor for feeding label printing sheets is performed. When a level difference between two adjacent output levels exceeds a predetermined value, it is determined that a top portion or an end portion of a label sheet is positioned in front of the transmission type sensor. According to this determination, the printing start position of the label sheet is positioned at a printing position at which a printing head is provided.
The positioning method as stated above is commonly used for a tag sheet, a label sheet and the like, whose back surface is printed with a black mark.
In this case, a reflection type sensor of an optical reflection type is used, and a non-black mark portion and a black mark portion can be recognized by the detection levels of the reflection type sensor. When the detection level varies largely at a boundary portion between a non-black mark portion and a black mark portion, it is determined that a top portion or an end portion of the black mark is sensed and the printing start position of the tag sheet or the label sheet is brought to the printing head position.
As has been explained above, since a conventional printer distinguishes a label portion and a gap portion, or a non-black mark portion and a black mark portion, a level comparison between two adjacent detection signals obtained at two steps is performed. However, the levels of the detection signals transiently or gradually varies at the boundary between a label portion and a gap portion or between a non-black mark portion and a black mark portion for several steps. Therefore, an error equivalent to several steps tends to occur when determination is made as to whether a detection position is a label portion or a mark portion, or a non-black mark portion or a black mark portion.
As a result, the top or end portions of the label sheet or those of the black mark portions cannot be detected accurately, which leads to a problem that positioning of the label printing sheets cannot be achieved at a high accuracy, thereby degrading the quality of printing, particularly, of color printing.